Bulk organic geochemistry of sediments from Puyehue Lake and its watershed (Chile, 40°S): Implications for paleoenvironmental reconstructions

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Abstract

Since the last deglaciation, the mid-latitudes of the southern Hemisphere have undergone considerable environmental changes. In order to better understand the response of continental ecosystems to paleoclimate changes in southern South America, we investigated the sedimentary record of Puyehue Lake, located in the western piedmont of the Andes in South–Central Chile (40°S). We analyzed the elemental (C, N) and stable isotopic (δ13C, δ15N) composition of the sedimentary organic matter preserved in the lake and its watershed to estimate the relative changes in the sources of sedimentary organic carbon through space and time. The geochemical signature of the aquatic and terrestrial end-members was determined on samples of lake particulate organic matter (N/C: 0.130) and Holocene paleosols (N/C: 0.069), respectively. A simple mixing equation based on the N/C ratio of these end-members was then used to estimate the fraction of terrestrial carbon (ƒT) preserved in the lake sediments. Our approach was validated using surface sediment samples, which show a strong relation between ƒT and distance to the main rivers and to the shore. We further applied this equation to an 11.22 m long sediment core to reconstruct paleoenvironmental changes in Puyehue Lake and its watershed during the last 17.9 kyr. Our data provide evidence for a first warming pulse at 17.3 cal kyr BP, which triggered a rapid increase in lake diatom productivity, lagging the start of a similar increase in sea surface temperature (SST) off Chile by 1500 years. This delay is best explained by the presence of a large glacier in the lake watershed, which delayed the response time of the terrestrial proxies and limited the concomitant expansion of the vegetation in the lake watershed (low ƒT). A second warming pulse at 12.8 cal kyr BP is inferred from an increase in lake productivity and a major expansion of the vegetation in the lake watershed, demonstrating that the Puyehue glacier had considerably retreated from the watershed. This second warming pulse is synchronous with a 2 °C increase in SST off the coast of Chile, and its timing corresponds to the beginning of the Younger Dryas Chronozone. These results contribute to the mounting evidence that the climate in the mid-latitudes of the southern Hemisphere was warming during the Younger Dryas Chronozone, in agreement with the bipolar see-saw hypothesis.

Introduction

The geochemistry of lake sedimentary organic matter generally provides important information that can be used to reconstruct paleoenvironmental changes in lakes and their watersheds. Total organic carbon (TOC) is comprised of material derived from both terrestrial and aquatic sources, and it is necessary to constrain these sources as well as possible for improving the interpretation of paleoenvironmental and paleoclimate records. A good understanding of the nature of the bulk sedimentary organic matter can also provide clues to interpret age-models based on radiocarbon measurement of bulk sediment samples (Colman et al., 1996). It is now commonplace to assess the origin of lake sedimentary organic matter using C/N ratios and carbon stable isotopes (e.g., Meyers and Teranes, 2001). However, to accurately reconstruct the relative contribution of each of the sources, it is essential to characterize these sources and look at the evolution of the geochemical properties of the organic matter during transport and sedimentation. This is however rarely done in paleoclimate and paleoenvironmental reconstructions.

Lake sedimentary organic matter is generally described as a binary mixture of terrestrial and aquatic end-members that can be distinguished by their geochemical properties. Aquatic macrophytes generally have C/N atomic ratios between 4 and 10; whereas terrestrial plants, which are cellulose-rich and protein-poor, produce organic matter that has C/N atomic ratios higher than 20 (Meyers and Teranes, 2001). Similarly, the carbon (δ13C) and nitrogen (δ15N) isotopic compositions of sedimentary organic matter have successfully been used to estimate the content of terrestrial and aquatic sources (Lazerte, 1983). In freshwater environments, however, the use of carbon and nitrogen stable isotopes is relatively limited because of the similar isotopic values for both the terrestrial and aquatic organic sources. The carbon and nitrogen isotopic composition of organic matter in lake sediments can however provide important clues to assess past productivity rates and changes in the availability of nutrients in surface waters (Meyers and Teranes, 2001).

One of the main questions in present-day paleoclimate research is the role of the Southern Hemisphere in the initiation of abrupt and global climate changes during the Late Quaternary. Several studies have demonstrated that climate records from Antarctic ice cores are clearly asynchronous with the rapid changes of the Northern Hemisphere, and suggest that abrupt paleoclimate changes are initiated in the Southern Hemisphere (Sowers and Bender, 1995, Blunier and Brook, 2001, EPICA Community Members, 2006).

Most of the paleoceanographic records available for the Southern Hemisphere follow a similar pattern, with sea surface temperatures of the Southern Pacific increasing in phase with Antarctic ice core records (Lamy et al., 2004, Kaiser et al., 2005, Lamy et al., 2007, Stott et al., 2007). What remains very controversial is the nature and timing of abrupt climate changes in the mid-latitudes of the Southern Hemisphere, especially in terrestrial environments (Barrows et al., 2007). In South America, currently available terrestrial records indicate either interhemispheric synchrony (Lowell et al., 1995, Denton et al., 1999, Moreno et al., 2001), asynchrony (Bennett et al., 2000, Ackert et al., 2008) or intermediate patterns (Hajdas et al., 2003).

Here, we present an integrated bulk organic geochemical study of the Puyehue lake watershed system (Chile, 40°S) to better understand the paleoenvironmental changes associated with climate variability in the mid-latitudes of South America. We investigate the bulk elemental and isotopic composition of the sedimentary organic matter deposited in the lake and its watershed to determine the sources of sedimentary organic matter and estimate their relative contribution through time. These data are then used to reconstruct paleoenvironmental changes in South–Central Chile during the last 17.9 kyr.

Section snippets

Location and setting

Puyehue Lake (40°40′S, 72°28′W) is one of the large glacial, moraine-dammed piedmont lakes that constitutes the Lake District in South–Central Chile (38–43°S; Campos et al., 1989). It is located at the western foothill of the Cordillera de Los Andes (Fig. 1) at an elevation of 185 m a.s.l.. The lake has a maximum length of 23 km, a maximum depth of 123 m and a mean depth of 76.3 m (Campos et al., 1989). It covers 165.4 km2 and is characterized by a complex bathymetry, with three sub-basins and

Terrestrial and aquatic sources

In order to constrain the terrestrial sources of sedimentary organic matter deposited in Puyehue Lake, we conducted a sampling campaign in the watershed of the lake in January–February 2002. Samples of living vegetation (V), soils (SP), paleosols (OC) and river sediment (RS) were collected at representative locations of the lake watershed.

Vegetation samples (V1 to V6) representing the six most abundant taxa were hand-picked from living plants and air-dried on the field. The selection of these

Particulate organic matter

The four lacustrine POM samples display C/N atomic ratios varying between 7.7 and 9.6 (8.5 ± 0.8) (average ± 1σ; Table 2, Fig. 3). The highest value is observed for sample F3, which is located near the mouth of the Golgol River, the main tributary and main source of detrital particles to the lake (Fig. 1). The lowest value is associated with sample F2, collected in the western sub-basin, and therefore protected from the direct influence of any river input. The δ13C values average − 28.0‰ (± 2.0). The

Sources of sedimentary organic matter

The interpretation of organic geochemical records of lake sediments requires an accurate understanding of the sources of organic matter. In lake systems, organic matter is generally a mixture of aquatic and terrestrial end-members in varying proportions (Meyers and Teranes 2001). These two groups can generally be distinguished by their C/N ratio because lacustrine algae are characterized by C/N values ranging from 6 to 12, while vascular land plants create organic matter that usually has C/N

Conclusions

The bulk organic geochemistry of sediments from Puyehue Lake and its watershed provides important information about the sources of sedimentary organic matter and changes in their relative contribution through space and time. We demonstrated that the C/N ratio of the potential sources of terrestrial organic matter in the lake watershed constantly decreases during incorporation into soils and transport to sedimentary environments. Therefore, the organic matter preserved in paleosols best

Acknowledgments

This research was partly supported by the Belgian OSTC project EV/12/10B “A continuous Holocene record of ENSO variability in southern Chile”. We acknowledge François Charlet for the collection of the POM samples and Elie Verleyen for stimulating discussions. Sediment cores were collected with the help of Fabien Arnaud, Christian Beck (University of Savoie, France), Vincent Lignier (ENS Lyon, France), Xavier Boës (University of Liège, Belgium), Waldo San Martin, and Alejandro Peña (University

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